3.2
MODELLING HORIZONTAL AND VERTICAL CONCENTRATION PROFILES OF OZONE AND NITROGEN OXIDES WITHIN AND DOWNWIND OF URBAN AREAS

James P. Nicholson, Univ. of Edinburgh, Edinburgh, Mid-Lothian, UK; and K. J. Weston and D. Fowler

The background tropospheric ozone concentration is depleted in the vicinity of urban areas due to the reaction of ozone with nitric oxide which is emitted by a range of urban sources. A model has been developed to simulate the three-dimensional structure in ozone and nitrogen oxides (NOX) concentrations in the boundary-layer within and downwind of urban areas. This is based on the boundary-layer Lagrangian column model of ApSimon et al (1994)*. The underlying photochemical processes of NOX and ozone, as well as emissions and deposition to the ground are simulated. The model treats the stability dependent vertical mixing in the boundary-layer with detail, dividing the boundary-layer into 33 levels, with greater resolution near the surface where the highest concentration gradients occur. The model simulates the diurnal variations in mixing using a combination of mechanical and convective turbulence.

The model is applied over a 100x100km domain, containing the city of Edinburgh (with a diameter of 12km), to simulate the city-scale processes of pollutants and to allow comparison with monitoring stations within and downwind of the urban area. Results are presented, using averaged wind-flow frequencies and appropriate stability conditions, as well as individual episodes, to show the extent of the depletion of ozone by a city. A landuse array has been created for input to the model with spatially and temporally variable emission and deposition values. The long-term average spatial patterns in the surface ozone and NOX concentrations over the model domain are reproduced quantitatively. One-dimensional trajectories through the city have been run under northerly and southerly wind regimes to allow comparisons to be made with data recorded from monitoring sites that lie on a north-south transect through the city - from an urban, through a semi-rural, to a remote rural location. Diurnal and annual (inter-seasonal) variations of ozone concentrations have also been modelled for the city-centre site, where the origin of the wind should not influence the extent of the ozone depletion.

* ApSimon, H.M., Barker, B.M., Kayin, S., (1994). "Modelling studies of the atmospheric release and transport of ammonia in anticyclonic episodes." Atmos. Environ. 28, pp665

The Second Symposium on Urban Environment